Questions

A: You may want to conceal from outsiders how rapidly objects
are being created. You may want specific formats that use less
characters than base10 but are human-friendly.

Q: Why not randomly generate identifiers?

A: If you want to avoid issuing the same id twice, you need to look
up each new id in a database or map. As the number of issued ids
grows, the number of lookups per new id grows. If you only plan
to use a tiny portion of the available keyspace, this is acceptable,
but inelegant compared to just encrypting an int.

Q: Why not just encrypt an integer with AES or something?

A: Because the output of the block cipher is a fixed size, which
is not likely to match your desired format.

A format of digits and uppercase letters, for instance, has 10**A * 26**B values.

The 128-bit outputs of AES would take 39 decimal digits to encode.

If you truncate the output of the block cipher, the ids are no longer
unique or reversible.